Widely used a rolls for rolling mills are compound rolls produced from cast iron by a centrifugal casting method. These compound rolls are those having a structure consisting of an outer layer made of a cast iron material in which a lot of highly wear-resistant carbides are precipitated, and an inner layer made of gray cast iron or ductile cast iron having large toughness. In this production method, however, only limited types of materials can be used for outer layers and inner layers.
Carbides of such elements as W, V, Nb, Ti, Ta, Zr, Hf, etc. have as high a Vickers hardness Hv as 2000 or more, and the inclusion of these carbides in outer layers is highly effective to increase the wear resistance of rolls. However, it is practically impossible to produce a compound roll comprising an outer layer precipitated with the above carbides and an inner layer strongly bonded thereto by a centrifugal casting method.
The reason for it is that since carbides formed from these elements have different specific gravities from that of a melt, they tend to be segregated due to a centrifugal action in the casting process. In addition, some of these elements are vulnerable to oxidation, making it extremely difficult to conduct melting, casting and bonding to the inner layer in the air. Further, in the centrifugal casting method in which gray cast iron or ductile cast iron containing graphite precipitates is used for the inner layer to provide it with sufficient toughness, when the outer layer contains a large amount of elements likely to form white cast iron, the formation of graphite in the inner layer is suppressed because of some components dissolved from the outer layer into the inner layer, making the inner layer brittle. In particular, since the carbides tend to be concentrated in a boundary between the inner layer and the outer layer, the boundary becomes brittle, making it likely that the outer layer peels off from the boundary.
In addition, when gray cast iron or ductile cast iron is used for the inner layer, the inner layer's tensile strength is at most 55 kg/mm.sup.2 or so, and its elongation is less than 1%. If higher tensile strength and elongation are to be obtained, the inner layer should be made of steel materials. However, this is difficult in the centrifugal casting method. This is due to the fact that since the inner layer has a higher melting point than the outer layer when steel is used for the inner layer, the outer layer is melted during the casting of the inner layer, thereby providing, as a final solidification phase, a boundary portion made of a mixture of both layers, this boundary portion being a site likely to generate cast defects.
Accordingly, the centrifugal casting method has been unable to provide a compound roll comprising an outer layer containing a large amount of carbides of the above-described elements and an inner layer having a tensile strength of 55 kg/mm.sup.2 or more and an elongation of 1.0% or more, both layers being strongly bonded to each other.
On the other hand, to increase the efficiency of rolling by increasing an amount of rolled products produced in a single roll repair cycle and to improve the size precision of rolled products, it has become necessary to drastically increase the wear resistance of rolls. At the same time, the improvement of size precision of rolled products makes it necessary to exert a bending force to both extending portions of a roll shaft in an opposite direction to the direction of deflection of the rolls caused by a rolling force, and the reduction of number of roll stands necessary to complete rolling makes it necessary to increase pressure applied to the rolls in a roll stand. This inevitably increases a bending force applied to both extending portions of the roll shaft, which in turn requires that the roll shaft has higher strength. However, in a roll in which an outer layer and a shaft are shrink-fitted or assembled, sliding may take place between the outer layer and the shaft, or the outer layer is susceptible to cracking during the rolling operation. Accordingly, the outer layer and the shaft should be completely metallurgically bonded to each other.
To satisfy these requirements at the same time, it is necessary that the outer layer is made of materials containing such components as to precipitate a large amount of carbides of elements such as W, V, Nb, Ti, Ta, Zr, Hf, etc., that the shaft is made of tough steel, and that the outer layer and the shaft are completely metallurgically bonded to each other. In this sense, casting methods such as the centrifugal casting method cannot be used for the above-described reasons. In addition, a diffusion bonding method between solid materials is not practically applicable because it needs extremely expensive facilities for relatively large objects such as rolls.
In view of the above circumstances, attention has been paid to a so-called shell casting method for manufacturing a compound roll by supplying a melt of an outer layer material around a central shaft made of cast steel or forged steel, bonding and solidifying the outer layer material to the shaft. Thus, compound rolls composed of various combinations of outer layers and shafts produced by the shell casting method have been proposed.
For instance, Japanese Patent Laid-Open No. 60-180608 discloses a compound roll for hot rolling having excellent resistances to adhesion of rolled materials and to surface roughening, which is composed of an outer layer and a shaft metallurgically bonded to each other, the outer layer being high-chromium cast iron containing, by weight, 2.0-3.5% C, 0.5-1.5% Si, 0.4-1.5% Mn, 8-25% Cr, 0.5-3.0% Mo, 10% or less V, 1.5% or less Ni and the shaft being made of cast steel or forged steel having a tensile strength of 55 kg./mm.sup.2 or more and an elongation of 1.0% or more. The outer layer is formed on the shell prepared in advance, by supplying a melt of an outer layer material onto the shaft and metallurgically bonding them. Bonding strength between the outer layer and the shell is higher than or equal to the strength of weaker one of the outer layer and the shaft, and the outer layer has a Shore hardness of 70 or more and, the decrease in its Shore hardness by a depth of 100 mm from its surface is 3 or less.
Japanese Patent Laid-Open No. 60-180609 discloses a high-chromium cast iron compound roll for cold rolling composed of an outer layer made of high-chromium cast iron and a shaft made of cast or forged steel with a surface hardness of 90 or more in Shore hardness, the outer layer being formed on the shell prepared in advance, by supplying a melt of an outer layer material onto the shell and metallurgically bonding it to the shell. The high-chromium cast iron of the outer layer consists essentially of, by weight, 2.5-3.5% C, 0.5-1.5% Si, 0.4-1.5% Mn, 0.5-3.0% Ni, 8-25% Cr, 1.0-5.0% Mo and balance substantially Fe, and the shaft has a tensile strength of 55 kg/mm.sup.2 or more and an elongation of 1.0% or more. Bonding strength between the outer layer and the shaft is higher than or equal to the strength of weaker one of the outer layer and the shaft.
Since the outer layers are made of high-chromium cast iron in both of the above compound rolls, their wear resistance is generally high, but it is not necessarily sufficient to satisfy the demanded level which is getting increasingly higher in recent years.
Japanese Patent Publication No. 51-24969 discloses an extremely wear-resistant steel having an iron matrix containing 0.2-0.6 weight % C and at least one selected from 2 weight % or less Ni, 2-6 weight % Cr, 1-6 weight % Mo, 1-6 weight % W and 10 weight % or less Co, in which carbides formed from 5-12 weight % V, 3-10 weight % Nb and carbon in an amount necessary for bonding to them are precipitated. It is described that this extremely wear-resistant steel can be used for rolls, but no description is given about its use for compound rolls.
In view of the above problems, an object of the present invention is to provide a wear-resistant compound roll composed of an outer layer made of a material in which hard carbides of W, V, Nb, Ti, Ta, Zr, Hf, etc. are precipitated, and a shaft made of tough steel, the outer layer free from the segregation of carbides and the shaft being metallurgically bonded to each other, and a method of producing such a wear-resistant compound roll in the air with relatively small cost.